Adding settings customization (#11)

@tonydspaniard @Ragazzo 
With this PR, we can modify yiinitializr-settings. Sample of [config](https://github.com/2amigos/yiinitializr/blob/master/config/settings.php) can be like the following:

```
$dirname = dirname(__FILE__);
$app = $dirname . '/../../..';
$root = $app . '/..';

return array(
	'custom' => array(
		'path' => $app . '/config/settings.php',
	),
	'yii' => array(
		'path' => $app . '/lib/vendor/yiisoft/yii/framework'
	),
	'yiinitializr' => array(
		'config' => array(
			'console' => $dirname . '/console.php'
		),
		'app' => array(
			'root' => $root,
			'directories' => array(
				'config' => array(
					'main' => $app . '/config',
					'console' => $app . '/config',
					'test' => $app . '/config'
				),
				'runtime' => array(
					$app
				),
				'assets' => array(
					$root . '/www'
				)
			),
			'classes' => array(
				'frontend' => array('\Yii', 'createWebApplication'),
			)
		),
	)
);
```

Custom config (sets in `custom` section) will be merged with the base config. Sample of such config can be like the following:
```
return array(
	'yiinitializr' => array(
		'app' => array(
			'directories' => array(
				'config' => array(
					'api' => $root . '/api/config',
				),
				'runtime' => array(
					'api' => $root . '/api'
				),
			),
			'classes' => array(
				'api' => array('\MyYii', 'apiApplication'),
			)
		)
	)
);
```